The present invention relates to layered surfaces and more particularly to a surface coating.
Molecular recognition and functional group complementarity are essential in the design and preparation of chemical or biological sensors,1 affinity chromatographic supports2 or in the build-up o organized supramolecular structures.3 New approaches to introduce molecular selectivity in these areas are thus of potential interest.
In this context organic thin films, formed by molecular self assembly, are presently being extensively studied.4 These can be prepared by the classical Langmuir Blodgett (LB) technique whereby a surfactant monolayer formed at the air water interface are transferred onto a solid flat surface, or by spontaneous sorption of an active surfactant onto a flat solid surface directly from solution. These processes lead to organized layers where the surfactants are held together by strong lateral interactions and stabilized by terminal covalent or polar bonds to the surface.
As model systems for chemical sensors the above-mentioned organic films present a number of advantages:
1) The high degree of order attainable in such systems allows a good control of the surface properties (polarity, hydrophobicity, acidity etc.) so that adsorption of a certain class of compounds can be either minimized (nonspecific protein adsorption) or maximized.5 
2) A number of signal transduction techniques are available (based on i.e. optical. electrochemical or microgravimetric measuring principles) allowing real time observation of surface processes.4 
3) Small sensing elements can be prepared using the lithographic technology available in the preparation of IC:s. Miniaturisation is an important factor in sensor design.
4) Surfaces can be rationally designed whereby analyte specific ligands or hosts are incorporated into the layers. This allows specific molecular recognition that can be monitored in real time.6,7,8. 
Simple chemical strategies to introduce selectivity are desirable from the aspects of stability and ease of preparation. Kunitake et al7 demonstrated that self-assembled guanidinium amphiphiles It the air water interface could be used for selective adsorption of adenosine-phosphates. The binding of ATP was believed to involve three guanidinium groups bound by hydrogenbonded ion pairs to the phosphate groups of ATP. Transferring thee layers by Langmuir-Blodgett techniques to a solid surface was suggested as a possible approach to sensor fabrication for phosphate biomolecules. However the limited stability of LB films are often a problem. The use of stable chemisorbed monolayers on flat surfaces is therefore becoming more important due to inherent advantages such as stability, ease or fabrication, order and miniaturization possibilities. Of these, particularly SAM:s formed by chemisorption of thiols on gold surfaces9 have been extensively studied. Exposing a Au (111) surface to a dilute alkylthiol solution results in rapid formation of a hexagonally packed all trans alkyl layer characterized by stable gold-sulphur bonds and a tilt angle between the gold surface and the alkyl chains of approximately 30xc2x0. A number of different functionalities can be chosen. In the fabrication of analyte selective interfaces the coatings are often irreversibly anchored to the sensor interface preventing regeneration of the coating. In the case of strongly bound analytes this may limit the reusability of the surface. Chemically selective coatings that can be reversibly attached to a sensor interface would in 1his context be of interest.
Multilayers with ordered structures are presently the focus of intense research due to emerging applications in optoelectronics (telecommunications), molecular electronics and chemical sensing.4,10 These can be achieved by incorporating functional units (dipoles, donor acceptor pairs, chromophores) into the hydrophobic part of the amphiphile. Due to stability problems of the resulting multilayers new alternative techniques need to be developed. The spontaneous self-assembly strategy is becoming increasingly popular. In this, usually two biofunctional building blocks, complementary to each other are repeatedly allowed to self-assemble on a solid substrate. In one such system, Decher et al described a strategy for multilayer formation based on consecutive adsorption of alternately charged bolaamphiphiles (amphiphiles containing two terminal polar groups) and polyelectrolytes.10d. This allows build-up of multiple layers with a total thickness of up to a few thousand xc3x85 thus giving the films bulklike properties. Furthermore these properties are ideal for non-linear optics since such a film would allow stable noncentrosymmetric orientation of polarizable dipoles. One goal in the construction of these films is to reduce the interlayer spacing and thereby to achieve a higher density of the functional units. Unfortunately this has a destabilizing influence on the formed layers. The building blocks (amphiphiles) are in these instances therefore larger than 30 xc3x85. Systems based on strong directed headgroup interactions would allow smaller amphiphiles to be used.
Routine gene analysis rely on the detection of specific DNA or RNA sequences present in minute amounts in a complex mixture. The current analytical methods usually involve time-consuming labelling and separation steps.11 and have therefore become a bottleneck in areas that depend on rapid DNA sequencing (i.e. HUGO, forensic analysis, diagnostics). Alternative methods for direct rapid sequencing are therefore being developed These often involve the use of presynthesized probe oligonucleotides capable of hybridizing specifically to the sequence of interest.12 An attractive approach is the direct monitoring (i.e. by optical, electrochemical or gravimetrical signal transduction) of the hybridization event using the probes attached to a solid surface (FIG. 3).13 Particularly intriuging is the combination of these methods with microlithography allowing the preparation of arrays of different probe sequences that each would represent a separate sensing element. In the above-mentioned systems however the probe is usually covalently linked to the support requiring additional chemical steps. Techniques for reversible attachment of the probe to the surface would be attractive for a rapid scanning of the hybridization properties of a large number of probes towards a certain target DNA.
The behaviour of proteins at interfaces and in particular at the solid/liquid interface is of outmost importance in determining the processes taking place upon contacting a surface with a biological fluid. Therefore, knowledge of the type of proteins adsorbed and their structure (conformation) is a field of intense research within the areas of implant integration, blood compatibility and dental plaque formation. A key issue is to be able to selectively adsorb the xe2x80x9cright proteinsxe2x80x9d in the desired conformation/orientation. Apart for varying solution conditions the major tool in this process is to tailor the surface with respect to functionality (type of groups and density). Techniques for quick and convenient control of these parameters would be a very useful instrument in optimize surface with respect to the above-mentioned applications. Similar considerations also applies for surface modification with respect to immobilization of enzymes for sensor applications.
The present invention provides a surface coating, characterized in that it comprises an amphiphile reversibly bound to a substrate by noncovalent interaction.
According to preferred embodiments:
The amphiphile is bound to the substrate by polar interaction between cationic groups of the amphiphile and anionic groups of the substrate;
The polar interaction between the amphiphile and substrate is pH dependent;
The amphiphile is a bolaamphiphile;
The amphiphile is selected from amidines;
The amphiphile is selected from bisbenzamidines;
The amphiphile is pentamidine;
The surface coating is used in chemical separation and analysis;
The surface coating is used in electronis; and
The surface coating is used in optoelectronics.
In a particularly preferred embodiment the amphiphile is a so-called bolaamphiphile (an amphiphile having two polar groups connected by a nonpolar linking group) selected from bisbenzamidines, preferably having a linking group with about 2-14 carbon atoms and particularly pentamidine, where one of the positively charged polar amidinium groups is reversebly bound to a negatively charged group, preferably a carboxylic group of a substrate by polar interaction. The reversibility of the binding of the amphiphile to the substrate is pH dependent and related to the pK-value of the acidic groups on the substrate. Usually this pK-value lies in the range from about 2 to about 6 and is about 4.5 for carboxylic acid. In the specific case of amidines, such as pentaamidine, bound to a substrate with carboxylic groups this means that substantially all of the amphiphile (amidine) is bound to the substrate at a pH of about 7-8.5 and is released from the surface at a pH of about 3 or below.
Generally, the present invention may be used inter alia in chemical separation and analysis, in electronics and in optoelectronics. By way of example some fields of use are:
Selective adsorption of biological molecules, such as phosphate biomolecules. Particularly, this selective adsorption nay be used in chemical sensors and detector devices; Sensors based on e.g. enzymes linked to the surface in an active orientation/conformation. Hereby the sensitivity/selectivity of the sensor could be optimized. The switching capacity could be utilized for quick regeneration or substitution of the type of immobilized enzyme for other target molecules;
Enhancing or inhibiting the compatibility with biological materials, e.g. inhibiting rejection in connection with surgery; enhancing the ingrowth of bone implants;
As a dental coating for inhibiting plaque growth or to improve adhesion of dental restorative materials;
As a matrix for binding phospholipids and provide models of biological membranes;
Inhibiting the coagulation of blood by coating a surface in contact with blood in accordance with the invention so that becomes compatible with the blood;
Administration of drugs by providing the surface of a carrier with a coating according to the invention where the amphiphile binds the drug and the bond is pH dependent so that the drug is released in dependence of the pH;
The invention may also be used o surface modify any administration vessel tubing or pump in order to minimize lose of active substance due to adsorption.
Another field of use is chromatography, more particularly column chromatography where the column or its packing is coated with the surface coating according to the invention. The coating includes an amphiphile (or a substance bound to the amphiphile) with affinity to the substance to be separated by chromatography. The chromatography process could e.g. constitute a method for purification of contaminated blood, where the contaminant is bound to the coating. As mentioned above, the affinity of the coating is preferably reversible and pH dependent so that the column can be easily regenerated;
A further field of use is extraction of substances from fluids, such as gold from seawater by providing the amphiphile with affinity to the substance (gold). Again, the affinity is preferably reversible (e.g. depending on the pH) so that the extraction device may be regenerated;
Still another field of use is the analysis of DNA sequences which will be described further below.